1. Field of the Invention and Introduction
The present invention refers to optoceramics, doped with activator elements, and having high transmissions, high densities and high effective atomic numbers. The activator elements are preferably chosen from the group of rare earth ions; titanium ions or transition metal ions are also possible. The materials are suitable to absorb high-energy radiation (preferably X-ray and gamma radiation as well as corpuscular radiation) and transform it to photons of visible light.
These materials are therefore, for example, suited as scintillator media for e.g. medical imaging (CT, PET, SPECT or combined PET/CT systems), security (X-ray detectors) or can serve in object tracing or investigation (exploration, prospecting for resources). The crystallite grains, forming the materials of the present invention, have cubic crystal structures (point and space groups as well as atom layers isotypic to those of the pyrochlore or fluorite minerals) or are clearly derivable from both mentioned minerals in terms of crystal structure.
In the present invention the term “optoceramic” refers to an essentially single phase, polycrystalline material of cubic symmetry and high transparency that is based on an oxide or other chalcogenide. Consequently, optoceramics are a special subgroup of ceramics. In this context “single phase” means that at least more than 95% of the material, preferably at least 97%, more preferably at least 99%, and most preferably 99.5 to 99.9% of the material is present in the form of crystals of the target composition. The single crystallites are densely packed and densities, in relation to the theoretical densities, of at least 95%, preferably at least 98%, and more preferably at least 99% are achieved. Hence, the optoceramics are almost free of pores.
Optoceramics differ from conventional glass ceramics in that the latter comprises a high proportion of amorphous glass phase next to the crystalline phase. Also, conventional ceramics do not have such high densities as optoceramics. Neither glass ceramics nor ceramics show the advantageous properties of optoceramics as represented by specific refractive indices, Abbe numbers, values of relative partial dispersions and, above all, the advantageous high transparencies for light in the visible and/or infrared wavelength regions.
Scintillator materials are active materials that absorb high-energy radiation directly or via a multitude of intermediate steps, wherein electron-hole pairs are generated. Their recombination leads to excitation of adjacent activator centers. The latter is thereby elevated into a metastable excited state. The relaxation of which leads, dependent on the choice of activator and host material, to emission of electromagnetic radiation in the energy range of near UV to near IR, i.e. 200 nm to 1500 nm, preferably 300 nm to 1100 nm (secondary radiation). This radiation is transformed into electric signals by suitable optoelectronic converters (photomultipliers or photodiodes). Areas of application are in the medical field (imaging and diagnostics), industrial inspection, dosimetry, nuclear medicine and high-energy physics as well as security, object tracing and exploration.
The requirements for detector materials for detection and conversion of high-energy radiation (X-ray and gamma radiation) to visible light are manifold:                high light yield and high energy resolution,        high transmission for secondary radiation (for coupling out the yielded visible light),        high X-ray or gamma radiation absorption efficiency,        low destruction or quenching of radiation        high chemical and refractive optical homogeneity,        good workability and true to form highly precise post-processibility of the scintillator material,        emission wavelength geared to the sensitivity of the detector,        short decay times, also for improvement of resolutions in time-of-flight experiments as well as for enabling faster scan velocities in order to keep the dose of radiation to the patient as low as possible, and        low afterglow after extinction of excitation radiation.        
Especially the aspects of high transmission as well as high X-ray and gamma radiation absorption cross sections are of extraordinary importance. Next, the material must be economically obtainable.
2. Description of the Related Art
Some CT-scintillators are known in the art, for example (Y,Gd)2O3:Eu (abbreviated “YGO”) and Gd2O2S:Pr,Ce,F (abbreviated “GOS”). Both are used in the form of ceramics. Single crystal growth of big individual crystals is not possible or extremely expensive due to the very high melting and breeding temperatures (above 2000° C.).
By sintering suitable powders, these compositions can be produced relatively cost-effectively at low temperatures significantly lower than 2000° C.
The problem with GOS material is its low symmetry of the crystalline phase (hexagonal arrangement of the crystallites). Because of the birefringence properties of each crystal grain in the densely sintered structure, any optical photon is subject to unwanted scattering. Highly transparent GOS ceramics are intrinsically not obtainable.
Eu:YGO, for example with the composition Eu:Y134Gd0.66O3 is as far as the density is concerned considerably more disadvantageous than GOS (about 5.92 g/cm3). It is thus worse than GOS concerning absorption of incident radiation. Additionally, GOS has a disadvantageously long decay time of about 1 ms (millisecond).
A sintered translucent ceramic for gamma ray imaging is described in U.S. Pat. No. 6,967,330. It has a stoichiometry of Ce:Lu2SiO5. However the crystal structure is not cubic and sintering ceramics with high transparencies is not possible even with very small crystallite grains (along the lines of GOS).
A layered ceramic of the composition Ce:Gd2Si2O7 (GPS) is described by Kamawura et al. (IEEE Conference 2008 Dresden 19. —Oct. 25, 2008, Proceedings, p. 67). It is especially suitable for detection of neutrons. The material was produced as a single crystal and then pestled to obtain a powder. The particle size is 50 to 100 μm. The material is not cubic and can thus not be sintered to transparent ceramics.
As a single crystal solution CdWO4 is still in use. However, this material has critically high cleavage properties and is thus only obtainable with difficulties and unreliably. Further, toxic cadmium is used during production.
In his lecture (TCCA-33) during the 4th Laser Ceramics Symposium (Nov. 10-14, 2008, Shanghai, China) J. Rabeau (Stanford University) described the production of transparent Ce:La2Hf2O7 (LHO) ceramics for scintillator applications by hot pressing. By hot pressing good transparencies could not be achieved; furthermore, the transparent ceramic is not stable due to the high lanthanum amount and decomposes after some time as it reacts with the water in the air.
Single crystals of Ce:Lu2Si2O7 (LPS) are described in Pidol, et al.: “Scintillation properties of Celu2Si2O7, a fast and efficient scintillator crystal”, J. Cond. Mat., 15 (2003), 2091-2102. These crystals have monoclinic symmetry; highly transparent ceramics are not obtainable. The material shows short decay times (38 ns) and low afterglow. However, light yield and energy resolution are only moderate.
A measure for the X-ray absorption capability of a scintillation host is the effective atomic number Zeff. The effective atomic number describes the average atomic number of a mixture of different substances. It can for example be calculated according to the following equation:
      Z    eff    =                              f          1                ×                              (                          Z              1                        )                    2.94                    +                        f          2                ×                              (                          Z              2                        )                    2.94                    +                        f          3                ×                              (                          Z              3                        )                    2.94                    +      …        2.94  wherein fn is the proportion of the total number of electrons that relates to the respective element and Zn is the atomic number of the respective element.
As a further index the product of the density and the fourth power of the effective atomic number Zeff is introduced. This index is proportional to the stopping power. Stopping power means the energy loss per wavelength unit of an incident particle, for example measured in MeV.
SELECTED SCINTILLATION HOSTS KNOWN INTHE ART HAVE THE FOLLOWING VALUES:Density,Typeg/cm3ZeffDensity × Zeff4 (×106)Y1.34Gd0.66O3ceramic5.924833Gd2O2Sceramic7.345991CdWO4single crystal7.9961111Gd3Ga5O12single crystal7.095043Lu2Si2O7single crystal6.236184
Malkin, Klimin et al. (Phys. Rev. B 70, 075112 (2004)) and Klimin (Phys. Sol. State, 47(8), 1376-1380, 2005) report titanium-containing single crystalline pyrochlore phases comprising rare earth ions on the A position. A variant of Yb3+:Y2Ti2O7 was produced as polycrystalline sample. The work focuses on single crystals, ceramics are described, too. However, these are produced at too low temperatures so that they cannot be transparent. The compositions are unfavorable for scintillator systems, because the emission wavelength of the Yb3+ ion is between 1000 nm and 1100 nm. The common optoelectronic converters in medical imaging system are not designed for such wavelengths.
Similar considerations apply to Schott's application DE 10 2007 022 048, wherein however only very small amounts of rare earth ions like Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er and Tm in the range <100 ppm are allowed due to the respective applications. Namely, it concerns passive ceramics.
In Ji, et al. “Fabrication of transparent HfO2 (40%)-Gd2O3:Eu ceramics from nanosized powders” (Electrochemical and Solid State Letters 8(7), H58-60, 2005) Eu-activated polycrystalline Gd2O3 is described which is stabilized by HfO2. The composition of the ceramics complies with Gd1.5Hf0.5O3.25=3Gd2O3*2HfO2, converted into molar proportions the composition is about 60 mol % Gd2O3 and 40 mol % HfO2. Its structure, however, is neither stably cubic nor isotypic to that of the pyrochlores at room temperature (defect structure derived from the fluorite structure). Potential application is in the field of medical diagnostics (CT detector).
So-called “transparent” ceramics of the composition La2Hf2O7 (LHO) are known from Ji, et al., “Fabrication of transparent La2Hf2O7-ceramic from combustion synthesized powders”, Mat. Res. Bull., 40(3), 553-559 (2005). Therein, powders of the target composition are used which had been synthesized by combustion reactions. The ceramics obtained thereby are at most translucent and free of rare earth ions.
It is clear from the state of the art that the currently described materials often do not have a highly symmetric cubic crystal structure (can therefore not be sintered to high transparency) and/or are in the form of a single crystal or layer that is not transparent. This is undesirable. As far as symmetric structures, if applicable also polycrystalline, are proposed they often do not satisfy the requirements of active material. As far as pyrochlore or fluorite structures are proposed at all they do not comply with current requirements. The variants that are known so far are either not transparent or only translucent and/or the density and/or the effective atomic number are too low or production is difficult. In case of La-containing forms the respective powders are additionally very hygroscopic and are only very difficultly convertible into transparent ceramics. Ceramics having pyrochlore structure and containing high amounts of Ti must be subjected to thermal post-processing in order to eliminate coloration by Ti3+ created in the reducing manufacturing process.